As an electrolyte material to be contained in a catalyst layer of a membrane/electrode assembly for a polymer electrolyte fuel cell, a fluorinated polymer having a cyclic structure and ion exchange groups has been proposed in view of excellent power generation properties of the membrane/electrode assembly (for example, Patent Document 1).
The fluorinated polymer having a cyclic structure and ion exchange groups is produced by converting, of a fluorinated polymer having a cyclic structure and ion exchange group precursor groups such as —SO2F groups, the ion exchange group precursor groups to ion exchange groups (such as —SO3−H+).
The fluorinated polymer having a cyclic structure and —SO2F groups may be produced, for example, by copolymerizing either one or both of a perfluoromonomer having a cyclic structure and a —SO2F group and a perfluoromonomer having a cyclic structure and having no —SO2F group, and tetrafluoroethylene (hereinafter sometimes referred to as TFE) and as the case requires, a perfluoromonomer having a —SO2F group and having no cyclic structure.
In a case where monomer components are polymerized to produce a fluorinated polymer, a mixture containing the fluorinated polymer, an unreacted monomer and a polymerization medium is obtained. Therefore, it is necessary to separate and recover the unreacted monomer and the polymerization medium from the mixture.
In order to recover the unreacted monomer and the polymerization medium directly from the mixture after the reaction efficiently, usually, the mixture is heated to a temperature sufficiently higher than the polymerization temperature under a pressure lower than the atmospheric pressure (for example, Patent Documents 2 and 3).